System Design

Question 1

name at least three design goals.

Answer 1

low cost, rapid development, min. # of errors, Reliability

Question 2

Name some typical trade-offs.

Answer 2

• Functionality vs. usability: e.g. complex menu
• Cost vs. robustness: ebay 2005 version
• Efficiency vs. portability
• Rapid development vs. functionality
• Cost vs. reusability: plastic bag vs. reusable bag
• Backward compatibility vs. readability -> design patterns

Question 3

What is a subsystem?

Answer 3

A subsystem is part of the system with a collection of closely related classes, associations, operations, events

Question 4

What is a Service/Subsystem interface?

Answer 4

A group of operations made externally visible and ready for use

Question 5

Describe Coupling and Cohesion. What is desired in software engineering (high/low Coupling/Cohesion)

Answer 5

➢ Cohesion measures dependency among classes (within a subsystem), preferably high (interaction within subsystems rather than across subsystem boundaries)
➢ Coupling measures dependency among subsystems, preferably low (calling class doesn ́t need to know about internals of called class – principle of information hiding, Parnas)

GOAL: High cohesion, low coupling

Question 6

Name 3 ways to deal with complexity.

Answer 6

• Abstraction
• Hierarchy
• Decomposition

Question 7

Shortly describe the layered architectural style.

Answer 7

Every layer is a subsystem that provides a service to another subsystem. Layers have no knowledge about higher layers and depend on lower level layers services.

Question 8

What is the difference between closed and open layered architectural styles? Shortly name the benefits of each.

Answer 8

In a closed architecture each can only call operations from a layer below. This leads to better maintainability, flexibility, portability, but less performance

In an open architecture a layer can call operations from any layer below. This leads to a higher performance and real-time operation support

Question 9

What does MVC stand for?

Answer 9

Model View Controller

Question 10

Describe the Model, View and Controller found in the MVC architectural style.

Answer 10

• View: subsystem containing boundary objects, display information to user
• Model: subsystem containing entity objects, application domain knowledge
• Controller: subsystem mediating between views, interact with user & notify views about changes in model, handle user input

Question 11

There are two ways for Notification in the mvc pattern. Which are those and how do they work?

Answer 11

• Pull notification: view retrieves data from model

* Push notification: model updates view after change

Question 12

Component and deployment diagrams! //TODO

Answer 12

A component diagram shows subsystems which provide and need certain interfaces

Question 13

Name two ways to persistently store data.

Answer 13

• File system: data used by multiple readers but a single writer
• Database system: data used by concurrent writers & readers